The present invention relates to a magnetic resonance imaging apparatus and magnetic resonance imaging method, and more particularly to a magnetic resonance imaging apparatus and magnetic resonance imaging method for correcting a tomographic image of an imaged region in a subject produced based on magnetic resonance signals emitted from the imaged region and received by a receive coil in an actual scan, with reference to a sensitivity distribution of the receive coil in the imaged region created in a reference scan.
Magnetic resonance imaging (MRI) apparatuses are widely used especially in medical applications because of their capability of capturing a tomographic image of a subject using the nuclear magnetic resonance (NMR) phenomenon.
In a magnetic resonance imaging apparatus, the subject is placed within a static magnetic field, whereby the direction of spins of protons in the subject is aligned with the direction of the static magnetic field to yield a magnetization vector, and electromagnetic waves of a resonance frequency are then applied to give rise to the nuclear magnetic resonance phenomenon in which the magnetization vector of the protons changes. Thereafter, the magnetic resonance imaging apparatus receives magnetic resonance (MR) signals generated when the protons recover their original magnetization vector, and produces a tomographic image of the subject based on the received magnetic resonance signals.
Receive coils for receiving the magnetic resonance signals in such a magnetic resonance imaging apparatus include a surface coil, which is often used because of its high S/N ratio and high sensitivity, and capability of preventing aliasing artifacts. The surface coil has, however, a property of decreasing reception sensitivity with increasing distance from the magnetic resonance signal source in the subject, and its sensitivity distribution is not uniform over the whole imaged region. Thus, there is a problem of a tomographic image produced based on magnetic resonance signals received by the surface coil becoming non-uniform.
In the prior art, a variety of methods are proposed to address this problem (for example, see Patent Document 1).
[Patent Document 1]U.S. Pat. No. 4,812,753, specification.
In the conventional techniques for addressing the problem caused by sensitivity non-uniformity of the surface coil, a reference scan is conducted in addition to an actual scan to calculate the sensitivity distribution of the surface coil in the imaged region, and a tomographic image produced by the actual scan is corrected based on the calculated sensitivity distribution.
In one of such conventional techniques, a reference scan is first conducted using a surface coil as a receive coil to image an imaged region in a subject according to a predefined imaging sequence, and a first reference image Is(x, y) is produced corresponding to pixel positions (x, y). Moreover, another reference scan is similarly conducted using a volume coil as a receive coil having a uniform sensitivity over the whole imaged region to image the imaged region in the subject according to a predefined imaging sequence, and a second reference image Ib(x, y) is produced. The first reference image Is(x, y) and second reference image Ib(x, y) are then subjected to processing for removing noise.
Next, as given by Equation (1) below, the first reference image Is(x, y) and second reference image Ib(x, y) are used to calculate a sensitivity distribution Hs(x, y) of the surface coil. The calculated sensitivity distribution H(x, y) is subjected to extrapolation or interpolation, and low-pass filtering in which lower frequency components are passed.Hs(x, y)=Is(x, y)/Ib(x, y)  (1)
Next, as given by Equation (2) below, a tomographic image Isa(x, y) produced in an actual scan using the surface coil is corrected by the sensitivity distribution Hs(x, y) to obtain a corrected tomographic image Isa′(x, y).Isa′(x, y)=Isa(x, y)/Hs(x, y)  (2)
Based on the corrected tomographic image Isa′(x, y), an image is displayed on a display section comprising a display device such as a graphic display.
In the conventional technique as described above, a plurality of reference images are captured using both the surface coil and volume coil prior to an actual scan, and the sensitivity distribution of the surface coil over the whole imaged region is calculated using the reference image by the volume coil. The calculated sensitivity distribution is then used to correct a tomographic image produced by the surface coil so that a uniform tomographic image can be obtained.
The volume coil, however, sometimes has a markedly non-uniform sensitivity distribution due to the permittivity effect especially when used in an ultra-high magnetic field of 3 Teslas or more, and at that time it cannot produce an adequately uniform image. Thus, when the sensitivity distribution of a surface coil is calculated using an image by the volume coil, the sensitivity distribution of the surface coil is sometimes inaccurately calculated.
Moreover, an open magnetic resonance imaging apparatus, for example, is sometimes not equipped with a receive coil having the high sensitivity uniformity of a volume coil or the like. In such a case, the sensitivity distribution of a surface coil over the whole imaged region as described above cannot be calculated beforehand, because there is no receive coil having a uniform sensitivity.
In the conventional techniques, as described above, the sensitivity distribution of the receive coil is sometimes inaccurately calculated, and it is sometimes difficult to capture an accurate tomographic image of a subject.